Iron is stored in the body in the form of the protein complexes, ferritin and hemosiderin, and is transported in the plasma via the protein complex, transferrin. It has been estimated that, under normal physiological conditions, about a third to a half of transferrin's iron capacity is utilized. Iron overload leads to saturation of the transferrin and ferritin and results in toxicity as the excess iron leaves the bloodstream and accumulates in tissues. In principle, this condition can be successfully treated by administration of known iron chelating agent which remobilizes the deposited iron and permits its excretion. In practice, none of the chelating agents which have been evaluated to date have proved entirely satisfactory, suffering from poor gastrointestinal absorption (oral inactivity) and either low efficacy or undesirable side effects.
A review of the clinical usefulness of iron chelating agents presently employed in the treatment of iron overload has defined the properties of the ideal drug. In summary, the drug should be inexpensive, orally administrable, non-toxic, and resistant to degradation prior to efficient absorption via the gastrointestinal tract. Once absorbed, it must be dispersed through the body by the bloodstream and able to bind avidly to iron in competition with transferrin. It should not interfere with intracellular iron biochemistry; e.g., cellular respiration, and should be resistant to metabolic changes which impair its iron sequestering ability.
Given current knowledge of the kinetics and thermodynamics of iron binding in vitro, of iron metabolism in vivo, and the ability to prepare chelating agents which bind iron (III) more effectively than the natural iron transport and storage proteins, it would seem a simple task to develop a drug capable of quantitatively removing iron from the body. The fact that this has not yet been achieved is a reflection of the other factors which determine drug efficacy, specifically their bioavailability and biostability; additionally, the toxicity of some drugs has restricted their utility. The molecular features which influence the bioavailability and stability of iron chelators and effect oral activity by optimizing GI absorption are noted in the esters of the present invention.
Most information on the relative effectiveness of iron chelators has been obtained using one of two animal models to simulate the condition of iron overload. Both animal models utilize IP injections of heat damaged red cells to achieve overload. In one screen utilized here, with results reported in Table 1, rat is the test animal, drug administration and red cell transfusions are concurrent, and efficacy is measured by the iron excreted in the urine and feces. The second screen uses the mouse, drug administration is initiated after transfusions are complete, and efficacy is based on percent iron depleted from the liver and spleen, plus urinary iron excretion.
On the basis of several related considerations, efforts were focused on the synthesis of polydentate chelating agents derived from phenol. Multidenticity was achieved by combining this ligand with amino and carboxylic acid ligands. The present chelating agents were evaluated in vivo using a hypertransfused rat or mouse screen. Two compounds reproducibly showed greater activity than deferrioxamine B (DFB), the current drug of choice for treatment of iron overload. These were EHPG and HBED. Some of their analogs shown below were also active. ##STR1## It was found that the alkyl esters of these dicarboxylic acids are very effective when administered orally. For example, the dimethyl and di-n-pentyl esters of HBED are more effective orally than is DFB administered I.P.